This study investigates friction and film thickness\nin elastohydrodynamic contacts of machined, rough\nsurfaces, where roughness is dominated by longitudinal\nridges parallel to the rolling/sliding direction. A ball-ondisc\ntribometer was used to simultaneously measure friction\nand film thickness in rough contacts as well as with\nnominally smooth specimens for comparison. The studied\nrough surfaces were selected so that the influence of the\nroot-mean-square roughness and roughness wavelength can\nbe assessed. Friction and film measurements were taken\nover a range of slideââ?¬â??roll ratios and speeds and with two\nlubricating oils with different viscosities, hence covering a\nwide range of specific film thicknesses. The measurements\nwith the nominally smooth specimens show that friction is\nstrongly influenced by thermal effects at high SRRs and\nthat the transition from mixed/boundary to full EHD\nlubrication occurs at lambda ratios greater than three. At\nlow speeds, the rough specimens are found to generate\nhigher friction than the smooth ones for all the roughness\nstructures considered, and this is shown to be related to the\nthinner minimum film thickness. Comparison of friction in\nrough and smooth contacts shows that the total friction in\nrough contacts can be divided into two components: one\nthat is equivalent to friction in smooth contacts under the\nsame conditions and is dependent on the slideââ?¬â??roll ratio,\nand the other that is due to the presence of roughness and is\nindependent of the slideââ?¬â??roll ratio under the conditions\ntested. Further analysis of the minimum film thickness on\ntops of roughness ridges indicates that even after the full\nlift-off, an effect of the roughness on friction persists and is\nmost likely related to the local shear stress in the micro-\nEHD contacts on the top of roughness ridges. At even\nhigher speeds, the difference in friction between the rough\nand smooth specimens vanishes.
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